The present invention relates generally to wireless devices for use in industrial process control systems. More particularly, the present invention relates to power conservation for field devices with built-in displays.
Process monitoring systems monitor and control process parameters in an industrial setting, such as pressure, temperature, flow, and level of process fluids used in industrial processes. For example, sensors coupled to transmitters are often employed at multiple locations in industrial manufacturing facilities to monitor and report a variety of process parameters along various production lines, while actuators coupled to receivers are used in other areas to, for instance, open and close valves in accordance with signals from a central control center.
Wireless devices are becoming prevalent in industrial applications. As components of wireless field device networks, wireless devices extend the reach of control or process monitoring systems beyond that of wired devices to locations where wiring may be difficult and expensive to provide. A wireless field device network includes of a cloud of wireless devices or nodes with a central controller or gateway. The nodes in the wireless network are able to both send and receive information.
Wireless field device networks are used to control and monitor disparate processes and environments. For example, wireless field device networks may be used in oil fields. An oil field is composed of numerous discrete locations centered on well pads that are scattered over large areas. Communication between these isolated local areas is essential to the overall management of the field. The wireless field device network at a well pad monitors and controls everything from flow rates and fluid temperature to valve status and position and potential leaks. The resulting data is relayed through the network to controllers that analyze the data and actuate control mechanisms in order to manage production or prevent trouble.
The term “field device” refers to any device mounted on industrial apparatus to perform a function in a process control system, including devices used in the measurement, control and monitoring of industrial plants, processes, or process equipment, including plant environmental, health, and safety devices. Each field device typically includes a sensor, an actuator, or both, and may perform a control or alert function. In wireless network systems designed for sensor/actuator-based applications, many devices in the network may be locally-powered because power utilities, such as 120V AC utilities or powered data buses, are not located nearby or are not allowed into hazardous locations where instrumentation, sensors, and actuators and safety monitors or human interface devices must be located without incurring great installation expense. “Locally-powered” means powered by a local power source, such as a self-contained electrochemical source (e.g., long-life batteries or fuel cells) or by a low-power energy-scavenging power source (e.g., vibration, solar, or thermoelectric). A common characteristic of local power sources is their limited energy capacity or limited power capacity, either stored, as in the case of a long-life battery, or produced, as in the case of a solar panel. Often, the economic need for low installation cost drives the need for battery-powered devices communicating as part of a wireless field device network. Effective utilization of a limited power source, such as a primary cell battery which cannot be recharged, is vital for a well functioning wireless field device. Batteries are expected to last more than five years and preferably last as long as the life of the product.
Some field devices incorporate a local operator interface (LOI) to facilitate maintenance and monitoring in the field. LOIs allow technicians to check process parameters and verify that field devices are working properly, in situ. A LOI may include a display such as a liquid crystal display (LCD). The power requirements of LOIs are modest, but are an important consideration for locally-powered devices, since continuously powering a display will unnecessarily drain a limited power supply. Because an LOI display may be needed only occasionally (e.g. while the field device is being locally checked by a technician), some field devices allow technicians to turn displays on or off with a button, thereby conserving power when the display is not in use.